...how is the inlet valve open ATDC - would not by then the valve be closed?
Hi Farrah, in a perfect world you would be correct on your assertion,
actually Loner is correct also, but doesn't seem to know why he is.
When a camshaft is ground, there are criteria chosen about
the rate of rise fall and an important thing called "Overlap".
A "Mild" cam in an engine will have a less aggressive rise/fall,
and of course a less aggressive overlap characteristic to it.
So that engine will idle smoothly and perform underwelmingly.
But I'm sure you've heard a gearhead's engine at some point,
it can hardly even idle because it has such aggressive overlap.
It is only efficient at one throttle setting, WEEEEEEEEEEEEEEEE...
Overlap is essential to the proper clearing of the combustion chamber,
as does the proper amount of restrictive back pressure from the exhaust.
Most people don't know that back pressure improves gas milage as an example,
and wonder why it plummets when they put on their fancy (IE: LOUD) custom mufflers.
So in reality, as the piston comes up with the exhaust valve open,
that exhaust valve is still open after it reaches TDC by a fair number of degrees,
as has also the intake valve been open from well before top dead center as well.
The flow and momentum out the exhaust pulls a small amount of the intake gas
out through the head area and into the exhaust manifold during this overlap,
and the waste spark lights that to help to finish burning more of the unburned fuel
from the horribly inefficient combustion engine's horrific implementation.
Poor octane values and low flame front speeds make gas and pistons a poor match,
but hydrogen has an excessively fast flame propagation front speed in comparison,
the fastest of all gases as far as I know...
So even though the momentum of flow out the exhaust
can flush a small amount of intake through the head
and out into the exhaust manifold for the waste spark to work.
The piston will be on It's way down creating a vacuum next
before the flame front can travel back into the intake manifold
causing traditional backfire issues one would expect
with fuels like gasoline (and other slow liquid/gaseous fuels).
Hydrogen is just to darn fast for that trick to work,
and it doesn't need a waste spark anyway
because it is done burning before the piston is even half way down.
On another note,
I think the K.I.S.S. principle has been overlooked here perhaps...
The need isn't for the waste spark to be eliminated via additional crap,
it is for the ignition timing to be obtained from the valvetrain instead,
not from the the crankshaft like is currently done on all (OEM) engines.
It can be mimicked with external gears off of the crankshaft,
but the timing information is already there on the camshaft.
any hall/magnet or coil/magnet fired ignition can be used,
just place the magnet on the camshaft in the first place.
And don't use a neo or other low currie-point magnet...
I will share this one personal point from experience,
I had a small (3.5-HP) motor that had no other use,
you know, the kind perfect to burn out with HHO...

I hand-ground the fall side of the exhaust lobe,
and then the rise side of the intake lobe until
there was no more overlap on the valvetrain.
I went simple and just put a set of points
inside the engine on the cam gear instead,
they overode and suppressed the OEM points,
thus eliminating the waste spark for the test.
The dramatic increase in stability using HHO
was remarkable enough to warrant sharing.
I additionally noted that a piece of paper held across the exhaust pipe
would alternate between being sucked tight against the exhaust pipe's end,
and puffing away to let the exhaust cycle from the piston evacuate the chamber.
In case you didn't catch that,
there was a vacuum upon initial exhaust valve opening...
Makes you wonder if there is a loss of power as the piston overcomes
the post-ignition contraction occurring in the chamber (apparently),
during the rest of the piston's travel downward towards BDC (?).
There may be a need for the cam's exhaust lobe
to have a large overlap on the rise side for HHO,
occurring well into the posr stroke to eliminate losses.
I'm not saying people should start grinding cams of course

,
but I can say with conviction that cams ground for HHO-use
are inevitably going to need to be produced and marketed for conversions.
Do you hear me Detroit ?!